| | | Bend Sensor® Variable Deflection Threshold Switch: This simple circuit is ideal for applications that require on-off switching at a specified deflection, such as touch sensitive membrane, cut-off, and limit switches. For a variation ofthis circuit that is designed to control relay switching, see Figure 1.6 on the next page. The Bend Sensor® device is arranged in a voltage dividerwith RM. An op-amp, U1, is used as a comparator. The output of U1 is either high or low. The non-inverting input ofthe op-amp is driven by the output of the divider, which is a voltage that increases with deflection. At zero deflection, the output of the op-amp will be low. When the voltage at the non-inverting input of the op-amp exceeds the voltage ofthe inverting input, the output ofthe op-amp will toggle high. The triggering voltage, and therefore the deflection threshold, is set at the inverting input by the pot R1. The hysteresis resistor, R2, acts as a "debouncer", eliminating any multiple triggering of the output that might occur. Suggested op-amps are LM358 and LM324. Comparators like LM393 and LM339 also work quite well. The parallel combination of R2 with R3 is chosen to maximize the desired deflection sensitivity range. A typical value for this combination is about 47 kQ. The threshold adjustment pot, R1, can be replaced by two fixed value resistors in a voltage divider configuration. | | |